In an effort to reduce polluting automobile emissions, a large segment of the automobile industry has been using emission control devices, such as catalytic converters on automobile exhaust systems. Such converters, through which the engine exhaust gases pass, are muffler-like devices which contain platinum-plated ceramic pellets or a monolith honeycomb material, which remove the hydrocarbons and carbon monoxide from the exhaust gases. Because of the rather high cost of the converter, it has been deemed necessary to fabricate the converter parts from an oxidation-resistant material so as to enhance its life span, in contrast to the rather short life of a conventional muffler. Because the exhaust gases are very hot, the fabricating material must also be a high temperature material, and for ease of construction should be weldable. The requirements for a weldable oxidation-resistant, high-temperature material has left designers with but one good choice, i.e. stainless steel.
Although stainless steel has been a most ideal material for use in fabricating catalytic converters, it is of course very expensive. To minimize cost, automobile manufacturers have been using the lowest cost stainless steel suitable for the purpose, i.e. AISI Type 409 stainless steel which contains about 11% chromium and 0.4% titanium. Emission control devices fabricated from Type 409 stainless steel have had extremely good service performance. The few failures that have occurred have been attributed to strength failures at elevated temperatures.
Because the extremely good service performance of present emission-control devices is an indication of overdesign, and because Type 409 stainless steel with its 11% chromium content is still very expensive, and because the supply of chromium is potentially unstable, automobile manufacturers would like to replace the Type 409 stainless steel with a lower cost steel having lower chromium contents, and hopefully having somewhat better high-temperature strength. Such a steel must however, have good formability and weldability comparable to Type 409 stainless steel. Although there are a number of stainless steel grades which contain appreciably lesser amounts of chromium than 11%, such grades have been unsuitable for use in emission control devices for one or more various reasons. For example, some high aluminum steels have been shown to have excellent oxidation-resistance at temperatures from 1400.degree. to 2200.degree. F. These steels, however, have rather erratic and hence unacceptable oxidation characteristics at temperatures of 1100.degree. to 1400.degree. F. More recently developed 3 to 6% chromium-aluminum steels have good oxidation-resistance at all temperatures up to 2200.degree. F. To impart any useful degree of ductility into these steels, however, the steels must be rigidly deoxidized and degassed or made by vacuum-melting techniques which very greatly increases the cost of the steel. Still other low-chromium-aluminum grades of steel, although suitably priced, are not readily weldable.